This research project investigates the mechanisms that activate the Drosophila Epidermal Growth Factor receptor (Egfr) during oogenesis, and the cellular pathways that mediate the response to this receptor tyrosine kinase in the ovarian follicle cells. Cell-cell communication plays an important role in the development of many tissues. Our model system focuses on signaling between the female germline and its surrounding follicle cells in the ovary of Drosophila melanogaster. We have shown that the Drosophila Egfr is expressed in the follicle cells and receives a highly controlled signal from the germline encoded by the gene gurken (grk). Restricted activation of the Egfr by Grk initiates several different follicle cell responses and is required for axis formation of the egg and embryo. We have also shown that grk expression in oogenesis can be regulated by a checkpoint mechanism. Problems in DNA repair during meiosis as well as other nuclear defects caused by retrotransposons, activate a meiotic checkpoint that controls translation of Grk in the oocyte cytoplasm. Our goal is to study the regulation of Gurken production in the germline and to analyze the patterning and differentiation processes that are activated in the follicle cells in response to receptor activation. Our specific aims are: 1) Checkpoint mediated control of Gurken production: 1A) The role of cutoff (cuff) in the piRNAi pathway. We have found that the gene cuff is a component of the piRNAi pathway that guards the germline against retrotransposon damage. Mutations in cuff activate a germline checkpoint. We will determine the molecular role of cuff in the piRNAi pathway and its effect on checkpoint activation. 1B) Translational regulation of gurken RNA. We will determine how the meiotic checkpoint regulates translation of grk. This will involve analysis of the gene vasa, as well as the translation initiation factor eIF1A that affects Grk protein levels in the checkpoint activated germline. 3) Analysis of the response pathway acting in the follicle cells of the ovary. We have defined several specific patterning responses to Egfr activation in the follicle cells. In particular, we have isolated mutations that affect posterior follicle cell differentiation and uncover interactions of Egfr signaling with the Notch pathway. We will analyze mutations in two genes that will allow us to describe the complex interactions of the two pathways in the posterior follicle cells. We will also compare the response of posterior follicle cells to that of dorsal follicle cells. Mutations in checkpoint genes, as well as unregulated activation of the human homologs of Egfr have been implicated in several forms of cancer. Our work will elucidate new roles of checkpoint genes, as well as analyzing the normal cellular pathways that regulate the activity of this receptor. It will also define downstream effector pathways operating in the follicle cell epithelium, a model system for epithelial development and differentiation.